Simultaneous infection with multiple strains of Mycobacterium tuberculosis

Drug-susceptible and drug-resistant isolates of Mycobacterium tuberculosis were recovered from 2 patients, 1 with isoniazid-resistant tuberculosis (pa tient 1) and another with multidrug-resistant tuberculosis (patient 2). An investigation included patient interviews, record reviews, and genotyping o f isolates. Both patients worked in a medical-waste processing plant. Trans mission from waste was responsible for at least the multidrug-resistant inf ection. We found no evidence that specimens were switched or that cross-con tamination of cultures occurred. For patient 1, susceptible and isoniazid-r esistant isolates, collected 15 days apart, had 21 and 19 restriction fragm ents containing IS6110, 18 of which were common to both. For patient 2, a s ingle isolate contained both drug-susceptible and multidrug-resistant colon ies, demonstrating 10 and 11 different restriction fragments, respectively. These observations indicate that simultaneous infections with multiple str ains of M. tuberculosis occur in immunocompetent hosts and may be responsib le for conflicting drug-susceptibility results, though the circumstances of infections in these cases may have been unusual.